Microstructure and Properties of Micro-Alloyed Mg-2.0Nd-0.2Sr by Heat Treatment and Extrusion

doi:10.1007/s40195-022-01474-8

Abstract

Abstract:

Magnesium (Mg) and its alloys have broad application prospects in the fields such as biomedical materials and automobile manufacturing. A micro-alloyed Mg-2.0Nd-0.2Sr (wt.%) magnesium alloy is designed and obtained through semicontinuous casting. The evolution of microstructures and tensile properties are investigated with different heat treatments and extrusion treatments. The grain sizes decrease significantly after extrusion, thus changing the fracture mode during the tensile testing process. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the properly processed extrusion alloy (referred to as MNS-E2) reach to 247 MPa, 228 MPa and 24%, respectively. The dramatical improvement of mechanical properties results from the refined grains and interactions between dislocations and precipitates. Some nanoparticle bands blocking the slippage and movement of dislocations are also found in the MNS-E2 alloy. The above causes combined result in an integrated effect of grain boundary strengthening, dislocation strengthening, precipitation strengthening and nanoparticle band strengthening. The contribution of strengthening mechanisms of MNS-E2 alloy consists of grain boundary with around 96 MPa, dislocations with around 3.4 MPa, precipitation strengthening with around 45 MPa and the nanoparticle band with around 18 MPa, respectively.

Key words: Mg-Nd-Sr alloy, Hot extrusion, Mechanical properties, Strengthening mechanisms

Zhenzhen Gui, Fan Jiang, Zhixin Kang, Fan Zhang, Zu Li, Jianhui Zhang. Microstructure and Properties of Micro-Alloyed Mg-2.0Nd-0.2Sr by Heat Treatment and Extrusion[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(2): 323-334.

Figures/Tables 15

Table 1 Detailed process of heating treatments for the as-cast Mg-2.0Nd-0.2Sr alloy

Sample ID	Treating process (first stage)		Treating process (second stage)
Sample ID	Temperature (°C)	Time (h)	Temperature (°C)	Time (h)
T4-1	540	6	-	-
T4-2	540	12	-	-
T6-1	540	12	200	24
H-1	200	12	430	24

Fig. 1 Schematic of the extrusion process

Table 2 Detailed pretreatment of the as-cast Mg-2.0Nd-0.2Sr alloy for hot extrusion

Sample ID	Pretreatment	Extrusion ratio (λ)	Post-treatment
MNS-E1	520 °C (10 h) + quenching	8.4	-
MNS-E2	520 °C (10 h) + quenching + 200 °C (24 h)	8.4	-
MNS-E3	200 °C (12 h) + 430 °C (24 h) + quenching	8.4	250 °C (10 h)
MNS-E4	200 °C (12 h) + 430 °C (24 h) + quenching	17.4	250 °C (10 h)
MNS-E5	540 °C (12 h) + quenching + 200 °C (24 h)	17.4	-
MNS-E6	540 °C (12 h) + quenching + 200 °C (48 h)	17.4	-

Table 3 Chemical compositions of the as-cast Mg-2.0Nd-0.2Sr alloys

Mg	Nd	Sr	Si	Fe
97.6 wt.%	2.07 wt.%	0.24 wt.%	0.01 wt.%	0.01 wt.%
99.57 at.%	0.35 at.%	0.07 at.%	0.01 at.%	-

Fig. 2 Optical microstructure of the as-cast Mg-2.0Nd-0.2Sr alloy at different magnifications

Fig. 3 SEM images of the as-cast Mg-2.0Nd-0.2Sr alloy at different magnifications

Fig. 4 SEM images of the Mg-2.0Nd-0.2Sr alloy at different heat treatment conditions: a T4-1 alloy, b T4-2 alloy, c T6-1 alloy, d H-1 alloy

Fig. 5 XRD analysis of the as-cast and heat-treated Mg-2.0Nd-0.2Sr alloys

Fig. 6 SEM images of different as-extruded Mg-2.0Nd-0.2Sr alloys: a, b MNS-E1, c, d MNS-E4, e, f MNS-E5, g, h MNS-E6

Fig. 7 Stress-strain curves of the as-cast and heat-treated Mg-2.0Nd-0.2Sr alloys

Fig. 8 Fracture characteristics of the as-cast and heat-treated Mg-2.0Nd-0.2Sr alloys: a as-cast alloy, b-d typical heat-treated alloy, e-g T4-2 alloy at different magnifications

Fig. 9 Stress-strain curves of the as-extruded Mg-2.0Nd-0.2Sr alloys with different treatments

Table 4 Tensile mechanical properties of the extruded alloys

Alloys	UTS (MPa)	YS (MPa)	EL (%)
MNS-E1	236 ± 7	211 ± 6	23 ± 3
MNS-E2	247 ± 3	228 ± 4	24 ± 3
MNS-E3	186 ± 5	168 ± 7	27 ± 5
MNS-E4	182 ± 7	170 ± 4	25 ± 4
MNS-E5	167 ± 9	136 ± 8	28 ± 4
MNS-E6	160 ± 7	135 ± 6	24 ± 3

Fig. 10 Fracture characteristics of the as-extruded (MNS-E3) Mg-2.0Nd-0.2Sr alloy: a-c at different magnifications, d EDS analysis of the broken eutectic compound

Fig. 11 TEM analysis of the extruded MNS-E2 alloy: a, c-f BF images with different characteristics, b SAED pattern of a eutectic compound particle in a

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